On the other hand, if the initial time is
chosen to be at the bounce, cautions must be taken on what initial conditions can be imposed consistently. In particular, if at this moment some modes are inside the comoving Hubble
radius and others are not, it is clear that in this case imposing the BD vacuum at the bounce will lead to inconsistencies.
In addition, there also exist the cases in which
particle creation in the contracting phase is not negligible, then it is unclear how a BD vacuum can be imposed at the bounce, after the universe is contracting for such a long time before the bounce.
Thus, in these cases other initial conditions need to be considered, such as the fourth-order adiabatic vacuum [1]}, [2]}, [3]}, [4]}, [5]}.
